This chapter wraps up various loose ends, by explaining who
Dobby's wizarding family
is and what their involvement with the
diary was.
Dobby also receives his
just reward.

Ignoring the elf bobbing apologetically around his ankles...

This arrogant, unconscious dismissal of servants - in particular, of an
enslaved servant whom Lucius
believes lacks the ability to resist his bondage - is one of the greatest
weaknesses in the security precautions taken by the
Malfoys and their like. This is
why Dobby was able to
find out so much: Lucius simply
didn't think to take into account that
Dobby was a witness to
his activities.

The same applies, of course, to Sirius
Black's relationship with
Kreacher, though
the tenor of that relationship differs from that between
Dobby and the
Malfoys.

...or Dumbledore announcing
that, unfortunately, Professor
Lockhart would be unable to return next year, owing to the fact that he
needed to go away and get his memory back. Quite a few of the
teachers joined in the
cheering that greeted this news.